The invention pertains to infrared light sensitive detectors and particularly to arrays of these detectors. More particularly, it pertains to infrared detector arrays incorporating associated electronics.
Sensitive and small infrared (IR) sensors or thermal detectors may operate on the principles of a thermopile or bolometer. A thermal detector may be an element which is sensitive to temperature changes and can produce an electrical output which may be used to measure changes of temperature of the element. One type of a thermal element may be pyroelectric detector which generates voltages and/or currents in response to changes in temperature. Such sensor may provide a small electrical signal which varies with the relative strength of the infrared radiation impinging on it. This sensor may be used to measure the temperature or change in temperature of an object on which a sensor of this type is focused. Another type of thermal detector, such as a bolometer, may be one whose passive electrical characteristics includes an electrical resistance that changes when the element is subjected to a temperature change. The most sensitive of these types of sensors may detect differences in temperature of a few thousandths of a degree Celsius in the object from which the infrared radiation emanates.
To be able to realize the most of the sensitivity of these infrared sensors, an effective layout, packaging and fabrication may be implemented. Using common photolithographic processes, such sensors may be fabricated in situ in a matrix or array, each sensor forming one of the pixels in the array incorporated in an integrated microcircuit. In general, an integrated microcircuit may have a variety of electrical components that are connected into a desired circuit. Such microcircuits may be made very small. To make such a device as an infrared sensitive solid state imaging device which is small in size, it may be advantageous to combine many thermal detectors within an integrated microcircuit. However, the combining of thermal detector elements within an integrated microcircuit may raise problems due to interfacing the detector elements and associated electronics such as readout circuitry which may need to be in close proximity to the elements to realize their sensitivities. One such problem may arise because the thermal detector elements need to be easily heatable or coolable to enable detection of low level thermal radiation. Therefore, for optimal performance, the thermal detectors may be thermally isolated from their ambient surroundings. That may preclude simply depositing or otherwise mounting the detectors directly on the microcircuit with the associated electronics because the semiconductor material is generally a good thermal conductor. Because of a thermal connection with the electronics, the thermal detectors may be thermally loaded or have much thermal inertia so as to adversely affect their performance in terms of sensitivity and speed.
An example solution to this problem may be a thermal detection device having numerous detectors formed on one surface of a semiconductor substrate. The other surface of the substrate may have one or more openings connecting the two surfaces. Electrical components may be formed on the other surface of the substrate to provide the supporting electronics for the detection device via the one or more openings. A layer of an electrical insulating material having a thermal resistance to the flow of heat along the plane of the layer may be on at least one of the surfaces. An example of a two level thermal sensor may be disclosed in U.S. Pat. No. Re. 36,136, reissued Mar. 9, 1999, entitled “Thermal Sensor”, by Robert E. Higashi et al., which is hereby incorporated by reference herein. Another example of a two level thermal sensor may be disclosed in U.S. Pat. No. 6,144,285, issued Nov. 7, 2000, entitled “Thermal Sensor and Method of Making Same,” by Robert E. Higashi, which is hereby incorporated by reference herein.
Many objects may emit various amounts of infrared radiation at wavelengths that may differ due to the emissivity, angle of the surface to the viewer, and temperature of the respective objects. Variations in this radiation when impinging on an array of infrared sensor elements may produce corresponding differences in the electrical signals output from the sensor elements in the array. Individual output signals from the sensor elements may be scanned in a sequential manner to form a composite signal collectively encoding an image of the field of view showing the objects which are the sources of the infrared radiation. The resulting image may be presented in real time and be used to form a visible image in a display which represents the spatial relationship of the objects in the field of view. The electronic circuitry may scan and amplify the signals from the individual sensor elements to provide a signal which may be used to reproduce the field of view on a screen.
Another example of an infrared sensor device, depending on a thermoelectric mechanism to provide the signal voltage output, may have thin layers of conductive materials of various types and insulating material which may be deposited in appropriate patterns on a silicon sensor substrate using photolithographic techniques. Thermoelectric junctions may be formed by overlapping conductors during the deposition. Such sensors may be referred to as microbridge sensors. The junctions of these microbridge sensors may be of two kinds, sensor junctions and reference junctions. The reference junctions may be in close thermal contact with the substrate. Each sensor junction may be within a small, discrete area which overlays a pit or depression formed in the sensor substrate, and may be of an area conforming to a footprint of the sensor junction. From a cross sectional view, these sensors may look much like a bridge spanning a valley, hence the term “microbridge”. The pits may provide a measure of thermal isolation from the substrate for their associated sensor junctions. Thus, changing infrared radiation impinging on both the sensor and reference junctions may result in the temperature of the sensor junction to change more rapidly than that of the reference junction, resulting in a temperature differential between the junctions which generates a signal. The photolithographic techniques may allow individual sensors to be fabricated in an array so as to allow imaging of the infrared radiation in a field of view. Leads from the elements forming the junctions may be led to electronic circuitry formed in another layer below the sensor substrate.
For maximum sensitivity, microbridge sensors may be maintained in a low pressure gas atmosphere or in a vacuum by virtue of reduced heat transfer between the sensing junction and the substrate, but this may require a hermetically sealed enclosure which adds cost and reduces reliability. It may also be possible to use a less tightly sealed enclosure containing air or other gas at or near atmospheric pressure, at the cost of less sensitivity. The microbridge sensor elements may be designed to produce a usable signal with a few hundredths or thousandths of a degree Celsius temperature differential between the sensing and reference junctions. One application for these sensors may be in arrays for forming images of relatively low contrast scenes or fields of view, such as may arise indoors in occupied rooms. In such fields of view, the inanimate, non-heat producing objects may all be very nearly at the same temperature. Distinguishing such objects by use of infrared imaging may require such sensitive sensors.
As noted above, infrared sensor elements in two dimensional arrays may be conventionally fabricated using two surfaces of a substrate or using a two-level structure in which the infrared absorbing or sensor elements are on a level different from the level of the associated and/or supporting electronics, such as monolithic readout circuits. Making the infrared detection system as a multi-level or multi-surface structure adds cost to the fabrication process. The present invention provides a significantly less costly, simpler and fewer-step single level approach with better yields, for fabricating an infrared sensor system having as good of performance, than other structural approaches such as the ones noted above.